This invention relates to electrochemical fuel cells, and, more particularly, to polymer electrolyte membrane fuel cells. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
The fuel cell converts chemical energy to electrical power with virtually no environmental emissions. A fuel cell differs from a battery in that it derives its energy from supplied fuel, as opposed to energy stored in the electrodes in the battery. Because a fuel cell is not tied to a charge/discharge cycle, it can in principle maintain a specific power output as long as fuel is continuously supplied.
One of the more commercially attractive types of fuel cells is the polymer electrolyte membrane (PEM) fuel cell. A single PEM cell consists of an anode and a cathode compartment separated by a thin, ionically conducting membrane. Hydrogen and oxygen (either pure or in air) are supplied to the anode and cathode compartments, respectively. The PEM prevents hydrogen and oxygen from directly mixing, while allowing ionic transport to occur. At the anode, hydrogen is oxidized to produce protons. These protons migrate across the membrane to the cathode and react with oxygen to produce water. The overall electrochemical redox (reduction/oxidation) reaction is spontaneous, thus, energy is released as well. When several "unit" cells are combined in a stack, higher voltages and significant power outputs can be obtained.
The advantages offered by a PEM fuel cell (e.g. its low operating temperature and non-liquid, non-corrosive electrolyte) make it attractive as a potential energy source for transportation and for portable and stationary power applications. Fuel cells have been successfully implemented in a number of utility, aerospace and military applications, but the high cost of fuel cells compared to conventional power generation technologies has deterred their potentially widespread commercial adaptation.
The high costs are primarily due to the catalyzed membrane assembly (CMA) and the bipolar/flow-field plates. The CMA consists of the PEM/conductive backing structure which typically utilizes exotic and expensive materials such as a platinum catalyst. Flow-field plates are commonly graphite or specially coated metal plates that have been machined to contain channels through which gas flow is directed across the plate. A bipolar plate has channels on each side to provide reactants to the anode and cathode of adjacent cells in a stack and may also incorporate some form of cooling channels. Stack manufacturers have historically used high-platinum loaded CMAs and intricately machined graphite bipolar plates, which have made the cost of a fuel cell much too expensive for most commercial applications.
An ideal bipolar plate would be a thin, light-weight, low-cost, durable, highly conductive, corrosion resistant structure that provides an effective flow-field configuration. The conventional flow-field design consists of a number of channels machined into a graphite or metal plate and is configured to provide relatively uniform reactant distribution combined with effective water removal. Until recently, achieving an effective flow-field design has come at the expense of tolerating thick and/or heavy bipolar plates with high material and machining costs.
The most commonly utilized bipolar plate material, graphite, is conductive and corrosion resistant, but it is expensive and not very durable due to its brittleness. Titanium has been used to a lesser extent. Though it is extremely hard and can be treated (e.g., nitrided) to provide adequate conductivity and excellent corrosion resistance, it is prohibitively expensive, heavy, and difficult to machine.
A number of technologies are being considered to replace machined bipolar plates in an effort to lower costs. Two of the more popular approaches considered are (1) the use of composite materials, such as the commercially available Kynar/graphite molded plates, and (2) relatively conventional flowfield/bipolar plate designs using mass production metal fabrication techniques in contrast to the piecewise machining currently being done. At present, these approaches have yielded only small incremental changes in fuel cell costs.
In a departure from conventional flow-field designs and fabrication techniques, U.S. Pat. No. 5,482,792 teaches the use of porous electroconductive collectors to distribute reactants and reaction products and to distribute electrical current to the electrodes. In one embodiment suitable collectors can be metal-wire fabrics or screens, wherein the wires form a series of coils, waves, or crimps, or other undulating contour. The collectors are situated within a gasket frame through which reactants are supplied to (and removed from) the collectors by a series of channels. These channels span the width of the collectors to attempt to evenly distribute reactants and reaction products. The gasket frame, which is made of a castable, elastomeric material (2 mm/0.079 in.) thick, seals against a metal frame, referred to in the patent as the bipolar plate. The bipolar plate separates adjoining cells within a stack and is also used for cooling the cells with which it is in contact. In the preferred configuration, the bipolar plate is made of aluminum and is necessarily thick (5 mm/0.197 in.) in order to adequately withdraw heat generated by the cell. Bipolar plates made of materials with lower thermal conductivities, such as stainless steel, are thinner (3 mm/0.118 in.), but have a more complicated design in order to accommodate extra channels for forced air cooling.
In addition to lowering or eliminating machining costs, another primary objective in bipolar plate design is to minimize thickness. While this may be of secondary importance for stationary power applications, the weight and size of the fuel cell stack has substantial implications in transportation applications. Minimizing bipolar plate thickness lowers stack weight, volume, and cost of materials, with a concomitant increase in the fuel cell power density. Stacks of individual unit fuel cells based on graphite hardware typically have low cell pitches (e.g., 1.6 cells/cm, 4 cells/inch) because the bipolar plates must be sufficiently thick to avoid cracking. Even though the stacks disclosed in U.S. Pat. No. 5,482,792 use metal components, a low pitch is still obtained because of the overall stack configuration.
Accordingly, it is an object of the present invention to provide a simpler and more effective reactant and cooling flow distribution configuration than provided by known bipolar plates.
Another object of the present invention is to provide uniform reactant distribution over a fuel cell membrane without machined bipolar plates.
Yet another object of the present invention is to provide thin, compact, and relatively light-weight cell "cartridges" that are mini-stacks containing two or more individual fuel cells, wherein adjacent cartridges are separated by a cooling plate when combined to form larger stacks.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.